The present invention relates to the field of microscopy and, more particularly, to the field of scanning tunneling microscopy.
The understanding of heterogeneous catalysis at a molecular level has been one of the central issues of physical chemistry for the past half century. Single crystal surfaces have served as valuable model catalysts providing insights into heterogeneous catalysis under vacuum conditions. This vacuum surface science approach of catalysis studies has revealed a tremendous amount of information for a great number of catalytic systems. However, industrial heterogeneous catalytic reactions are usually carried out at high pressure and high temperature. There are considerable experimental challenges in the high pressure studies, which are necessary in order to understand molecular behavior under realistic conditions. The potential difference in adsorption, surface structures, and catalytic mechanisms between the model studies at low pressure and industrial reactions at high pressure is often referred as pressure gap.
A key component in studying the pressure gap is to characterize the adsorbed layer of the reactant gases at high pressure during catalytic processes. A simple extrapolation of the insights into the adsorption structure obtained at low pressure and low temperature is not necessarily applicable to high pressure and high-temperature conditions which could have different energetic pathways. Thus, to obtain a complete understanding of catalysis, it is necessary to perform studies of surface catalytic reactions under high pressure of reactants.
Scanning tunneling microscope (STM) has the unique capability of studying catalyst surfaces atom by atom, which is invaluable for elucidating the adsorption structure and the mobility of reactant molecules during catalysis. This technique can be applied in a pressure range from UHV (ultra high vacuum) to atmospheric or higher pressure since the tunneling process between the sample and tip only occurs in a very close range of 5-50 Å. It has been applied to catalytic studies under a condition of relatively high pressure by a few groups (see B. J. McIntyre et al., Rev. Sci. Instrum. 64, 687 (1993); J. A. Jensen et al., J. Vac. Sci. Technol. B17, 1080 (1999); P. B. Rasmussen et al., Rev. Sci. Instrum. 69, 3879 (1998); E. Laegsgaard et al., Rev. Sci. Instrum. 72, 3537 (2001); A. Kolmakov et al., Rev. Sci. Instrum. 74, 2444 (2003); and M. Röβler et al., Rev. Sci. Instrum. 76, 023705 (2005)) since the first demonstration by McIntyre et al. High pressure studies have been performed of STM by filling reactant gases into an STM chamber connected to the UHV preparation chamber (McIntyre et al.; and Jensen et al.). However, this method has disadvantages such as large volume of reactant gases and limits in sample heating, reactant gas pressure, and spatial resolution.